The long-term objective of our research is to determine the role of the CRES (Cystatin-Related Epididymal Spermatogenic) protein in epididymal sperm maturation and sperm function. We have established that CRES defines a new subgroup in the family two cystatins of the cystatin superfamily of cysteine protease inhibitors. Although predicted to structurally resemble the archetype family two cystatins, we have recently demonstrated that CRES does not function as a typical cystatin. We determined that CRES does not inhibit cysteine proteases but rather is a competitive inhibitor (Ki = 25 nM) of the serine protease prohormone convertase 2 (PC2). Because PC2 is a member of a family of proteases with critical roles in the proteolytic maturation of prohormones and proproteins, we propose that CRES may be a new convertase inhibitor that mediates proteolytic processing events important for reproductive function. My short-term goals are to establish whether CRES interacts with PC2 or other convertases in vivo as well as to examine CRES as a convertase inhibitor in greater detail and specifically examine its effects on proprotein processing in cells. Other short-term goals are to examine the biological roles of CRES in vivo including its function in the sperm acrosome and fertilization. My long-term goals are to contribute to our understanding of how proproteins are activated to their mature and functional forms, specifically within spermatozoa and the epididymis, and the mechanisms by which these processes are regulated. Ultimately this may provide mechanisms to target for male contraceptive development. The RCDA, if awarded, will assist me in accomplishing my goals. Specifically, it will release me from Departmental obligations including substantial teaching in medical school gross anatomy, thereby allowing me to devote my full attention to research. In addition, with this award I will not be required to serve on additional committees both at the institutional and departmental levels. The Department of Cell Biology and Biochemistry provides a supportive environment for the development of my research career. Within the department is a large group of NIH-funded reproductive biologists who provide expertise and a collegial, interactive working environment. Also, the newly purchased mass spec will enable on site protein analysis for the studies described above. In short, the environment at TTUHSC is exceptional and will fully support my proposed studies.